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The Importance Of Risk Factors In The Control Of Leishmaniasis
1 Nov 2007
Source: WHO/TDR
Alejandro Llanos-Cuentas
Facultad de Salud Pública y Administración Universidad Peruana Cayetano Heredia Lima, Peru
Working paper for the Scientific Working Group meeting on Leishmaniasis Research, convened by the Special Programme for Research and Training in Tropical Diseases, Geneva, 2–4 February 2004
Full text source: Scientific Working Group, Report on Leishmaniasis, 2–4 February 2004, Geneva, Switzerland, Copyright § World Health Organization on behalf of the Special Programme for Research and Training in Tropical Diseases, 2004, http://www.who.int/tdr/publications/publications/swg_leish.htm 
The leishmaniases are vector-borne diseases, and are highly complex: they are usually zoonotic; a large number of Leishmania species infect humans; the biological cycles and epidemiological interrelationships between vector, reservoir, humans, climate and ecology are highly variable; they have multifaceted clinical manifestations (due to the genetic heterogenicity of both parasite and humans) and outcomes (which vary from spontaneously cured to not curable) as a consequence of disease.
The magnitude of leishmaniasis has changed little in the last 50 years in some endemic areas of the New World. For instance, in some areas of Peru endemic for cutaneous leishmaniasis (CL), the prevalence during the 1990s was similar to the prevalence before the 1950s (Davies et al., 1994); although the DDT house spraying campaign between the 1950s and 1970s reduced the annual incidence of leishmaniasis, when spraying was stopped around 1972, CL rose again until it had reached its traditional level. The global incidence and prevalence of leishmaniasis increased generally in the Americas between the 1980s and 1990s, despite the intervention measures and efforts of the health authorities. Other vector-borne diseases such us malaria, dengue, yellow fever, and bartonellosis have increased too. The growth of the population, poor levels of hygiene, and increasing periurban poverty are creating favourable habitats for the proliferation of vectors and reservoirs. As a consequence, vector-borne disease such us malaria, dengue, and lymphatic filariasis are becoming major public health problems [9]; the same is now happening with leishmaniasis, which is rapidly becoming urbanized, as reported from Manaus and Belo Horizonte [14].
The contribution that scientific knowledge is making to New World leishmaniasis is indisputable. A large number of papers have been published since the early 60s, including detailed reviews of parasitological, ecological, entomological, immunopathological, clinical, therapeutic and public health aspects. Before the 1980s, the majority of studies were observational in nature, but since the 1980s, analytic research has increased, looking more at causality, pathogenic mechanisms, transmission patterns and the interrelationship between vectors, reservoirs, humans, climate and ecology. Research is necessary to provide solid information for improving intervention measures and prevention programmes; however, despite the important questions raised, only a few scientific results have been incorporated into control programmes, and these have been without significant impact for the people living in endemic areas.
Table 1 summarizes the intervention targets recommended for leishmaniasis by the WHO Expert Committee [18]. These actions have been applied frequently, partially and independently of each other. The most common intervention is early diagnosis and treatment (action against the parasite), but this does not reduce transmission. The vector is usually controlled (inside houses and in the peridomiciliary and sylvatic environments) only in outbreaks, with limited benefit in the long term, while control of the animal reservoirs (domestic and wild) is restricted to special situations. Personal protection measures (which reduce man-fly contact) are used only on a small scale and in time-limited situations, usually by tourists or personnel employed by extraction companies working in the jungle. A vaccine is an important alternative, but is still in the experimental stages.
Table 1
Intervention targets in leishmaniasis
Vector control | ||
∞ | Insecticide spraying | |
∞ | Genetic control | |
∞ | Biologic control | |
∞ | Chemical control | |
Control of animal reservoirs (domestic and wild) | ||
∞ | Drugs | |
∞ | Vaccine | |
Personal protection | ||
∞ | Vaccine | |
∞ | Repellents | |
Early diagnosis and treatment | ||
These actions have not so far been successful in controlling leishmaniasis in the New World. The weakness of the strategy is the biomedical approach, the application of interventions through vertical programmes, the lack of sustainability in the long term, and the low or absent participation of people from endemic areas. The approach using risk factors is an alternative to this classical approach.
In the Latin American literature, several factors have been associated with a high risk of transmission. These include occupational activities mainly related to deforestation, extraction of natural products, hunting and exploring [13,3]. Age (young adults), sex (male), and low socioeconomic status are factors closely related with these occupations in the jungle areas of Brazil, Peru and Mexico [1–2,8]. The location of homes close to the forest has also been related to increased risk of disease [3,12]. All of these risks have been detected on the basis of statistical association, but are not representative (in terms of sample size) or adjusted for multifactorial determinants; however, the results have provided a hypothesis for analytic studies. As a disease very closely associated with poverty, leishmaniasis usually affects the poorest people of the poorest countries; 72 of 88 countries affected are developing, and 13 of them are among the least developed. It is here, where more than 80% of the population earns less than US$ 2 per day (Davies et al., 2003), that the majority of cases are reported every year [7].
Most properly designed assessments of risk factors were published after 1990. Risk factors associated with the transmission of Leishmania infections have been demonstrated in Peru [11,6], Colombia [17], Costa Rica [15], and Argentina [16,19], among other countries; they vary in importance and nature according to region, epidemiological and ecological characteristics, and pattern of transmission. The risk factors are quite different if the transmission is in dwellings (indoors or peridomestic) or outside the home (in forests or rural places). In addition, there is often more than one pattern of transmission in the same region. For instance, in the Andean regions where indoor transmission has been classically recognized, peridomestic and rural transmission occur at the same time. In some endemic areas, transmission around the home causes about 80% of cases; in other Andean places, where transmission occurs in rural places [10], a single Leishmania species (L. peruviana) may cause more than 90% of the CL cases. A similar situation occurs when the main transmission pattern is in the forest; however domestic transmission is always more frequent [17].
The studies about risk factors suggest that ecologic and climatic characteristics, and human, vector and reservoir behaviours, together influence the transmission pattern, vector species, abundance of vectors, rate of infection of vectors and reservoirs, and incidence and prevalence of human infection. The frequency of the disease, its severity and rate of cure are more dependant on the host-parasite relationship and the antiparasitic action of the drugs used.
Despite the fact that risk factors have been determined in several countries, they have been used very little. Only in one study [11] in Peru was estimation of the population attributable risk (PAR) for an individual subset of factors simultaneously adjusted for the risk attributable to the remaining factors in the model. A group of risk factors in central West Peru (region 1) was significantly associated with transmission inside houses because the suggested intervention was to use a single measure (spraying insecticide indoors). The combined PAR for this group was 0.792, which implies that removal of this group of factors with a specific intervention would lead to a 79.2% reduction in CL incidence in the region. In order to evaluate the impact of the same intervention measure in North-West Peru (region 2), a group of risk factors related to indoors transmission was selected and the best PAR was found to be only 5.1. For five years, both regions were systematically studied (spraying lambda-cyhalothrin in the home and peridomestic area every six months for two years, and measuring the incidence rate for an additional three years); reduction in the incidence of CL in region 1 was 81%, but in region 2 was only 4.6%. Thus, the risk assessment approach allows the results of studies to be drawn upon for prioritizing the use of limited resources and concentrating efforts against those hazards with greater impact.
The studies on risk factors have taught us several lessons:
Leishmaniasis has a dynamic epidemiology; factors can change under the influence of climate, ecology and behaviour of humans, vectors and reservoirs.
We cannot apply the same intervention strategy in all places, even when the infection is caused by a unique species.
There are limitations of the control strategy used in the majority of countries. The classical endpoints of control programmes are control of the disease (early diagnosis and treatment) and control of the vector(s) or reservoir(s), but in fact the primary endpoints are climate, ecology and human behaviour.
In future, leishmaniasis control programmes in developing countries should be focused on reduction of poverty and development of the health rights of the people in order to achieve sustainable success through control activities. So health and economic improvement must be critical issues in the national policy. The community should be adequately educated, in a long-term programme, about the strategies. Thus, a combined risk assessment and ecosystem approach is a new alternative based on the interrelationship between scientific knowledge, participation of the communities living in endemic areas, and economic support by the government.
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